Cooling systems and methods

ABSTRACT

A cooling system for a server may include an evaporator that is in thermal communication with the server so that the evaporator absorbs heat generated by the server. An inlet end of a condenser is fluidically connected to an outlet end of the evaporator, whereas an outlet end of the condenser is fluidically connected to an inlet end of the evaporator. A working fluid disposed within the evaporator and the condenser is substantially free of lubricant. The working fluid absorbs heat generated by the server in the evaporator, changing from a liquid phase to a vapor phase. The working fluid releases heat in the condenser, changing from the vapor phase back to the liquid phase. The phase changes of the working fluid in the evaporator and the condenser result in pressure changes sufficient to cause the working fluid to circulate between the evaporator and the condenser in a self-sustaining, phase-change thermodynamic cycle.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application No. 61/377,791, filed on Aug. 27, 2010, which is hereby incorporated herein by reference for all that it discloses.

TECHNICAL FIELD

The present invention relates to cooling systems in general and more particularly to cooling systems and methods for cooling servers located in data centers.

BACKGROUND

Data center cooling systems are known in the art and are commonly used to cool computer servers and other electronic equipment located within data centers. Such cooling systems are configured to maintain specific temperatures and humidity levels to prevent the servers and other electronic equipment from overheating and malfunctioning. Typical data centers use complex computer room air conditioning (“CRAC”) systems. In general, CRAC systems intake room temperature air from the data center and discharge cold air into the data center to maintain the desired temperature range and humidity level. However, such air conditioning units have high power demands and are relatively inefficient, leading to increased operational costs. Indeed, in many cases, the cooling systems consume as much, if not more, energy than the computer equipment itself.

As an alternative to conventional air conditioning systems, data center designers have proposed various types of low energy, non-refrigerant-based cooling systems. Examples of such low energy, non-refrigerant-based systems include “airside” and “waterside” economizers. While such systems can be made to work in many applications, they are not without their own problems. For example, airside economizers are perceived as introducing various air quality issues, such as the introduction of corrosives and dust/dirt. It is also difficult to maintain a consistent humidity level with airside economizers. Waterside economizers carry with them risks associated with water piping and associated leakage issues and also suffer problems associated with the operation of cooling towers during freezing weather. In addition, waterside economizers consume considerable amounts of water.

Consequently, a need remains for an improved cooling system for computer and data centers that does not suffer from the problems associated with current systems. Ideally, such a system should be capable of providing effective cooling for computer systems, but without the high energy demands of refrigerant-based systems. Additional advantages could be realized if such an improved system solved the problems associated with airside and waterside economizer systems.

SUMMARY OF THE INVENTION

A cooling system for a server according to one embodiment of the present invention may include an evaporator having an inlet end and an outlet end. The evaporator is in thermal communication with the server so that the evaporator absorbs heat generated by the server. An inlet end of a condenser is fluidically connected to the outlet end of the evaporator, whereas an outlet end of the condenser is fluidically connected to the inlet end of the evaporator. A working fluid disposed within the evaporator and the condenser is substantially free of lubricant. The working fluid absorbs heat generated by the server in the evaporator, changing from a liquid phase to a vapor phase. The working fluid releases heat in the condenser, changing from the vapor phase back to the liquid phase. The phase changes of the working fluid in the evaporator and the condenser result in pressure changes sufficient to cause the working fluid to circulate between the evaporator and the condenser in a self-sustaining, phase-change thermodynamic cycle.

In another embodiment, a cooling system for a server having a heat discharge member may include an evaporator having an inlet end and an outlet end that is in direct thermal communication with the heat discharge member of the server. A condenser having an inlet end and an outlet end is located at a higher elevation than the evaporator. A supply conduit connects the outlet end of the condenser to the inlet end of the evaporator. A return conduit connects the outlet end of the evaporator to the inlet end of the condenser. A working fluid disposed within the evaporator, the condenser, the supply conduit, and the return conduit enters the inlet end of the evaporator in a liquid phase and absorbs heat from the heat discharge member thereby transitioning the working fluid from the liquid phase to a vapor phase. The working fluid in the vapor phase exits the outlet end of the evaporator and travels though the return conduit before entering the inlet end of the condenser, wherein the working fluid in the vapor phase discharges heat thereby transitioning the working fluid in the vapor phase to the liquid phase before traveling through the supply conduit back to the evaporator.

Also disclosed is a method for cooling a server that includes the steps of: Evaporating a working fluid contained in an evaporator in thermal communication with the server, the working fluid being substantially free of lubricant, the evaporating producing a working fluid stream in a vapor state; supplying the working fluid stream in the vapor state to a condenser; condensing the working fluid in the vapor state to produce working fluid stream in a liquid state; and returning the working fluid stream in the liquid state to the evaporator, wherein phase changes of the working fluid in the evaporator and the condenser result in pressure changes sufficient to cause the working fluid to circulate between the evaporator and the condenser in a self-sustaining, phase-change thermodynamic cycle.

Another method may involve the steps of: Supplying a working fluid in a liquid phase to an evaporator in direct thermal communication with the heat discharge member of the server; discharging heat from the heat discharge member of the server to the evaporator; transferring the discharged heat from the heat discharge member to the working fluid in the liquid phase, wherein the working fluid in the liquid phase absorbs the discharged heat; transitioning the working fluid into a vapor phase; supplying the working fluid in the vapor phase from the evaporator to a condenser located at a higher elevation than the evaporator, wherein the working fluid in the vapor phase is supplied from the evaporator to the condenser by a pressure differential caused by the condensation of the working fluid in the condenser; cooling the working fluid in the vapor phase with the condenser; transitioning the working fluid in the vapor phase to the liquid phase; and supplying the working fluid in the liquid phase back to the evaporator.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative and presently preferred exemplary embodiments of the invention are shown in the drawings in which:

FIG. 1 is a pictorial diagram of one embodiment of a cooling system for cooling servers provided in a server rack;

FIG. 2 is an enlarged side view in elevation of the server rack, heat discharge member, and evaporator that may be utilized in one embodiment to form the direct thermal communication between the heat discharge member and evaporator;

FIG. 3 is a rear view of the server rack and evaporator arrangement of FIG. 2 more clearly showing the relative positioning of the heat discharge member and evaporator; and

FIG. 4 is a pictorial diagram of another embodiment of the cooling system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of a cooling system 10 according to the teachings of the present invention is illustrated in FIG. 1 as it may be used to cool a plurality of servers 24 located in a data center 20. Briefly, but as will be described in much greater detail herein, the cooling system 10 involves a self-sustaining, phase-change thermodynamic cycle 12 that does not require the use of separate pumping apparatus to move a working fluid 40 within the system 10. Instead, the working fluid 40 circulates within the system 10 due to pressure differences resulting from fluid phase changes occurring within the system 10.

The cooling system 10 may comprise a condenser 50 and an evaporator 60 that are fluidically connected together. More particularly, a supply conduit 74 may be used to fluidically connect an outlet end 54 of condenser 50 with an inlet end 62 of evaporator 60. A return conduit 76 may be used to fluidically connect an outlet end 64 of condenser 60 with an inlet end 52 of condenser 50. In the particular embodiment illustrated in FIG. 1, the condenser 50 is located at an elevation that is higher than an elevation of evaporator 60, as depicted schematically in FIG. 1. However, other arrangements are possible, as will be described in further detail herein. In addition, the evaporator 60 in the embodiment illustrated in FIG. 1 is arranged so that it is in direct thermal communication with the heat discharge member 26 of servers 24. As used herein, the term “direct thermal communication” means that substantially all of the heat (represented by arrows 28) discharged from heat discharge member 26 of servers 24 is conducted directly to the evaporator 60, i.e., without passing through an intermediate medium (e.g., open room air) and without substantial heat transfer to such intermediate medium. Alternatively, other arrangements for the evaporator 60 possible, as will also be described in further detail herein.

In operation, the condenser 50 provides the working fluid 40 in a liquid phase 44 to the evaporator 60 via supply conduit 74. In the evaporator 60, heat 28 discharged from the heat discharge member 26 of servers 24 is absorbed by the working fluid 40. The heat absorbed changes the working fluid 40 from the liquid phase 44 to a gas or vapor phase 42. Thereafter, the working fluid 40 in the liquid phase 44 (i.e., as a working fluid stream in the vapor phase) is conducted to the condenser 50 via the return conduit 76. In the condenser 50, heat (represented by arrows 58) from the working fluid 40 in the vapor phase 42 is released into a suitable cooling medium (e.g., air or water). The amount of heat released causes the working fluid 40 to re-condense into the liquid phase 44. The working fluid 40 in the liquid phase 44 (i.e., as a working fluid stream in a liquid state) is then supplied back to the evaporator 60 via supply conduit 74 to complete the self-sustaining, phase-change thermodynamic cycle 12.

The phase changes of the working fluid 40 occurring in condenser 50 and evaporator 60 result in pressure changes that are sufficient to cause the working fluid 40 to circulate between the condenser 50 and evaporator 60 in a self-sustaining manner. More specifically, the vaporization of the working fluid 40 in the evaporator 60 results in an increase in the vapor pressure of the working fluid 40. The increase vapor pressure of the working fluid 40 in the evaporator 60 is greater than the vapor pressure of the condensing working fluid 40 in the condenser 50. The resulting pressure difference causes the working fluid 40 to flow within the system 10, thereby dispensing with the need, at least in certain embodiments and operating at certain temperature regimes, to provide separate pumps or compressors to move the working fluid 40.

Because the present invention does not require the use of pumps or compressors, the working fluid 40 may be substantially free of lubricants. In addition, there is no need in the present invention to achieve any particular minimum velocity for the various working fluid streams (e.g., the working fluid stream in the vapor state leaving the evaporator 60 and the working fluid stream in the liquid state leaving the condenser 50), in order to provide a sufficient flow of lubricant to a pump or compressor. To the contrary, it is generally preferred to design the cooling system 10 of the present invention so that the velocities of the various working fluid streams are below a level at which pressure head losses would prevent the working fluid 40 from circulating through the cooling system 10 in the self-sustaining manner described herein. In one embodiment, such velocity reductions may be achieved by providing the system 10 with fluid conduits (e.g., condenser and evaporator coils 56 and 66, as well as the supply and return conduits 74 and 76) having sufficiently large diameters or cross-sectional flow areas.

Moreover, even in embodiments that may utilize a pump or compressor (typically on an as-need basis only), the pressure-head increase that may need to be provided by such a pump or compressor will still be significantly below the pressure head required in conventional, refrigerant-based systems. Consequently, even in embodiments utilizing pumps or compressors, the cooling system of the present invention will still require much less energy during operation compared to conventional, refrigerant-based systems.

Still other advantages may be associated with the particular working fluid 40 selected as the heat transfer medium. In one embodiment, the working fluid 40 is selected so that the required phase changes (e.g., from liquid to vapor and back again) occur at moderate pressures, typically on the order of atmospheric pressure or slightly above atmospheric pressure. The moderate pressures involved with such a working fluid 40 dispense with the need to provide expensive, high-pressure tubing, conduits, evaporators, condensers, and related equipment, thereby further reducing the cost and complexity of the cooling system 10 compared to conventional, refrigerant-based cooling systems.

For example, in many embodiments, the cooling system according to the present invention may utilize any of a range of non-metallic tubing or piping for the supply and return conduits 74 and 76. Similarly, the condenser 50 and evaporator 60 need not be constructed or rated to work with the high working pressures typically associated with conventional server cooling systems. The installation cost of the cooling system 10 is therefore significantly reduced relative to traditional refrigeration-based cooling systems.

Yet other advantages of the cooling system 10 may be realized in embodiments in which there is direct thermal communication between the heat discharge member 26 of server 24 and the evaporator 60 of cooling system 10. For example, the direct thermal communication between these two elements or components substantially reduces the amount or quantity of heat 28 from servers 24 that is released into the environment of the data center 20, thereby significantly reducing the heat load on the data center cooling system 20. Indeed, in many embodiments, the data center cooling system 20 need not be provided with any additional cooling capacity, other than that which normally would be required for a room of comparable size but without the added heat load from the servers 24.

Having briefly discussed one embodiment of the cooling system 10 according to the present invention, as well as some of its more significant features and advantages, various exemplary embodiments and alternative configurations of the cooling system will now be described in greater detail. However, before proceeding with the detailed description, it should be noted that while the cooling system 10 is shown and described herein as it could be used in a particular application, e.g., to cool server systems, and having a particular configuration, the present invention could be used in any of a wide variety of other applications and configurations, as would become apparent to persons having ordinary skill in the art after having become familiar with the teachings provided herein. Consequently, the present invention should not be regarded as limited to the particular applications and configurations shown and described herein.

Referring now primarily to FIGS. 1-3, one embodiment of the cooling system 10 is shown and described herein as it may be used to cool one or more servers 24 provided in a data center 20. The various servers 24 may be provided in one or more server racks 22, although only a single server rack 22 is shown in FIGS. 1-3. Each of the servers 24 provided in server rack 22 may include a heat discharge member 26 through which is discharged heat 28 produced by various electrical components 29 of server 24. More specifically, and in one exemplary arrangement, each server 24 may be provided with one or more fans 30 provided at a position near the back portion 36 of server 24, adjacent the heat discharge member 26, as best seen in FIG. 2. In operation, the fan or fans 30 draw air 31 through one or more vents 32 positioned at a front portion 34 of the server 24. Within the server 24, air 31 is drawn over electrical components 29. Heat 28 from the electrical components 29 is absorbed by the air 31 as it passes through the server 24. The heat 28 (i.e., contained in heated air stream 31) is thereafter discharged via the heat discharge member 26 located at the back portion 36 of the server 24. See FIG. 3.

The evaporator 60 of cooling system 10 should be positioned in thermal communication with the server 24 so that evaporator 60 can carry-away the generated heat. Generally speaking, it is preferred, but not required, to position the evaporator 60 at a location where it will be exposed to the highest temperatures produced by the server 24. So positioning the evaporator 60 will maximize the evaporator pressure and minimize the need to provide additional circulation/condensation assistance (e.g., via a separate pump or compressor system), even with high cooling air (i.e., condenser) temperatures.

With the foregoing considerations in mind, in one embodiment, the evaporator 60 of cooling system 10 may be positioned with respect to the server 24 so that it is in direct thermal communication with the heat discharge member 26 of server 24. Such a mounting arrangement will allow heat 28 discharged from the heat discharge member 26 to be transferred directly to the evaporator 60, with only minimal amounts being lost to the environment (e.g., data center 20). That is, instead of being transferred to ambient air or environment of the data center 20, as is the case with conventional server cooling systems, the heat 28 discharged from the heat discharge member 26 is transferred substantially directly to the working fluid 40 contained within evaporator 60. The direct thermal communication between the heat discharge member 26 and evaporator 60 may be accomplished in one embodiment by mounting the evaporator 60 to the back of the server rack 22, i.e., directly adjacent the heat discharge members 26, by screws 61 or other fasteners.

In another embodiment, the evaporator 60 may comprise a housing configured to encapsulate or enclose the heat-producing components (e.g., processors) of the servers 24. In such an embodiment, the working fluid 40 will be in direct contact with the processors or other heat-producing components of the servers 24. The encapsulation of the heat producing components will generally expose the working fluid 40 in the encapsulating evaporator 60 to higher temperatures, e.g., in the range of about 48° C. to about 60° C. (about 120-140° F.), compared embodiments wherein the evaporator 60 is in thermal communication with cooling air 31 discharged by the server fans 30. Such higher temperatures will generally result in substantially higher evaporator pressures that, in most cases, will allow the working fluid 40 to circulate within the system 10 without the need for additional pumps or compressors, even with cooling air temperatures (i.e., for condenser 50) in excess of 38° C. (about 100° F.)

Referring back now to FIG. 1, in an embodiment wherein the evaporator 60 is in direct thermal communication with the heat discharge member 26 of server 24, the evaporator 60 may comprise an inlet end 62 and an outlet end 64 that are fluidically connected by one or more coils 66. A plurality of fins or micro-channel heat transfer elements (not shown) may be thermally associated with the coils 66 to increase the surface area of the evaporator coils 66, thereby enhancing the transfer of heat 28 to the working fluid 40 contained within the coils 66. Alternatively, other configurations for the evaporator 60 are possible, as would become apparent to persons having ordinary skill in the art after having become familiar with the teachings provided herein.

Evaporator 60 may be fabricated from any of a wide range of materials (typically metals and metal alloys) having a high thermal conductivity, such as aluminum, copper, and various alloys thereof, although other materials may be used. Significantly, however, and depending on the particular working fluid 40 that is used, evaporator 60 need not be constructed to withstand the high working pressures typically associated with conventional refrigerant-based cooling systems.

Cooling system 10 may also comprise a condenser 50. In one embodiment, condenser 50 may comprise an inlet end 52 and an outlet end 54 that are fluidically connected by one or more coils 56. As was the case for evaporator 60, condenser 50 may also be provided with a plurality of fins or micro-channels (not shown) to increase the heat transfer surface area of the condenser coils 56, thereby enhancing the transfer of heat 58 from the working fluid 40 contained within the coils 56. Alternatively, other arrangements for the condenser 50 are possible, as would become apparent to persons having ordinary skill in the art after having become familiar with the teachings provided herein.

Condenser 50 may be fabricated from any of a wide range of materials (typically metals and metal alloys) having a high thermal conductivity, such as aluminum, copper, and various alloys thereof, although other materials may be used. Significantly, however, and as was the case for evaporator 60, condenser 50 need not be constructed to withstand the high working pressures typically associated with conventional cooling systems.

In one embodiment, condenser 50 is mounted at an elevation that is higher or greater than the elevation at which is mounted the evaporator 60. So mounting the condenser 50 at an elevated location will assist in returning the working fluid (i.e., in a liquid state or phase 44) to the evaporator 60. In many embodiments and applications, mounting the condenser 50 so that it is in an elevated position with respect to evaporator will substantially reduce or even eliminate (in certain circumstances and operational regimes) the need to provide a pump or compressor system (not shown in FIG. 1) to assist in the circulation of the working fluid 40. In an exemplary installation, the condenser 50 maybe provided on a roof (not shown) or other elevated location on a building (also not shown) housing the data center 20.

The outlet end 54 of condenser 50 may be fluidically connected to the inlet end 62 of evaporator 60 by a supply conduit 74. Similarly, the outlet end 64 of evaporator 60 may be fluidically connected to the inlet end 52 of condenser 50 by return conduit 76. Supply and return conduits 74 and 76 may be substantially identical to one another, if desired, although they need not be. They may also be fabricated from any of a wide range of materials metallic and non-metallic materials, such as metals or plastics, that would be suitable for the particular application. In selecting the particular materials and other specifications for the conduits 74, 76, it should be noted that, in many embodiments, the operating pressures of the cooling system are considerably below the operating pressures of conventional, refrigerant-based cooling systems. Consequently, the conduits 74, 76 need not be capable of withstanding high pressures. In accordance with the foregoing considerations, then, in one embodiment, the conduits 74 and 76 may be fabricated from polypropylene. Use of polypropylene will allow any joints or junctions in the conduits 74, 76 to be conveniently made by means of heat fusion, thereby substantially reducing the likelihood of leaks.

In considering the design of the system 10, and in particular the sizes and configurations of the condenser 50, condenser coils 56, evaporator 60, evaporator coils 66, as well as the supply and return conduits 74 and 76, it should be noted that it will be generally preferred to design the system 10 so as to reduce the velocities of the various working fluid streams (i.e., the working fluid stream in the vapor state exiting the evaporator 60 and the working fluid stream in the liquid state exiting the condenser 50) below a level at which pressure head losses would prevent the working fluid 40 from circulating through the system 20 in a self-sustaining manner. In one embodiment, such a velocity reduction may be achieved by increasing the diameters or flow cross-sectional areas of the various flow conduits involved. Besides lowering the velocities of the various working fluid streams, such large area conduits will permit significant mass flow rates of working fluid 40, thereby increasing the overall heat transfer rate of the system 10.

The working fluid 40 of the cooling system 10 should comprise a working fluid having phase change temperatures (i.e., vaporization and condensation temperatures) in a range suitable for the expected operational temperature ranges of the condenser 50 and evaporator 60. Moreover, and ideally, those phase change temperatures should occur at moderate pressures, thereby avoiding the need to provide the system 10 with components and systems that are capable of withstanding high operating pressures. Additional advantages could be realized by selecting a working fluid 40 wherein the pressure increases rapidly with relatively small increases in temperature.

Generally speaking, good results can be obtained by utilizing a working fluid 40 having a boiling point of less than about 18° C. (65° F.) at about 1.01 bar (1 atm). Additional advantages may be realized by selecting a working fluid 40 that also has the following characteristics: A low freezing point (lower than about −109° C. (−165° F.) at about 1.01 bar (1 atm)); a low vapor pressure (less than about 0.172 bar (25 psia)); a high vapor heat capacity (greater than about 0.8 kJ/kg K (0.2 Btu/lb ° F.) at about 1.01 bar (1 atm)); a low liquid viscosity (less than about 413 Pa s (1 lb/ft hr)); a low vapor viscosity (less than about 20.7 Pa s (0.05 lb/ft hr)); high liquid thermal conductivity (greater than about 0.07 W/m K (0.04 Btu/hr ft ° F.)); and high vapor thermal conductivity (greater than about 0.009 W/m K (0.005 Btu/hr ft ° F.)). In addition, it is generally preferred that the working fluid 40 be substantially free of lubricant. Providing a substantially lubricant-free working fluid 40 will improve the heat transfer performance of the system 10.

By way of example, in one embodiment, the working fluid may comprise a propane-based halocarbon fluid, such as 1,1,1,3,3-pentafluoropropane (R-245fa), which is readily commercially available.

The cooling system 10 may be operated as follows to remove heat 28 from servers 24. Working fluid 40 in a liquid phase or state 44 enters the inlet end 62 of evaporator 60. The working fluid 40 in the liquid phase 44 absorbs the heat 28 from the heat discharge member 26. The working fluid 40 continues to absorb the discharged heat 28 until it reaches a boiling point and begins to transition into the vapor phase or state 42. The working fluid 40 in vapor phase 42 ultimately exits the evaporator outlet 64 as a working fluid stream in the vapor state. During the vaporization process, the vapor pressure of the working fluid 40 increases beyond the vapor pressure of the condensing working fluid 40 in condenser 50. This pressure difference will cause the working fluid stream in the vapor state to flow into condenser 50. If conduits of sufficient diameter are provided, or of the system 10 is otherwise designed with minimal flow restrictions, the pressure head loss resulting from the flow of working fluid 40 in the system 10 will be less than the pressure differential resulting from the phase changes of the working fluid 40. The working fluid 40 will then flow through system 10 in a self-sustaining manner.

In the particular embodiment shown and described herein, the temperature of the air discharged from the heat discharge member 26 of server 24 is at a temperature of about 40° C. (about 104° F.). Working fluid 40 is provided in sufficient amounts to cool the discharged air to a temperature roughly equal to the ambient air in the data center 20, about 24° C. (about 75° F.) As explained above, in an embodiment wherein the working fluid 40 comprises R-245fa, a discharge temperature of about 40° C. will raise the vapor pressure in the evaporator 60 to a level sufficient to allow the working fluid 40 to circulate within the system 10 in an self-sustaining manner, even with moderately high cooling air temperatures at the condenser 50. Moreover, in an embodiment wherein the evaporator 60 encapsulates the heat-producing components (e.g., processors) of server 24, or is otherwise exposed to temperatures in excess of 40° C. (104° F.), e.g., temperatures in the range of about 48° C. to about 60° C. (about 120° F. to about 140° F.), such excess temperatures will result in even higher evaporator pressures that will allow the working fluid 40 to circulate in a self-sustaining manner even when the cooling air temperature at the condenser 50 exceeds 38° C. (100° F.)

After leaving the evaporator 60, the vaporized working fluid 40 enters the condenser 50, whereupon it is condensed. The condensation process is accomplished by releasing or rejecting heat from the vaporized working fluid 40 in an amount sufficient to cause the working fluid 40 to condense. In the particular embodiment shown and described herein, the working fluid 40 enters the inlet end 52 of condenser 50 at a temperature of about 40° C. (about 104° F. or higher in some embodiments), in the vapor state 42. Condensed working fluid 40 (i.e., in the liquid state 44) is discharged from the condenser 50 (e.g., via outlet end 54) at a temperature of about 29° C. (about 85° F.). The condensed working fluid 40 then flows to the inlet end 62 of evaporator 60 whereupon the thermodynamic cycle 12 is repeated.

As mentioned, the working fluid 40 circulates through the evaporator 60 and through the rest of the cooling system 10 at relatively low pressures. In the particular embodiment shown and described herein, the pressure of the working fluid 40 within the evaporator 60, condenser 50, return conduit 76, and supply conduit 74 does not typically exceed about 2.07 bar (about 30 psia), and is generally at about 1.7 bar (about 25 psia). However, in embodiments wherein the working fluid 40 is exposed to even higher temperatures (e.g., in a range of about 48° to about 60° C.) in the evaporator 60, the system pressure will still not normally exceed about 4.1 bar (about 60 psia). In contrast, refrigerant-based systems may exceed pressures of 20.7 bar (300 psia), nearly 10 times the pressure at which the present invention operates. The ability of the cooling system 10 to circulate working fluid 40 at low pressures allows for relatively inexpensive low-pressure rated components to be used for the various elements (e.g., condenser 50, evaporator 60, supply conduit 74, and return conduit 76) comprising the cooling system 10. Moreover, if a pump is required or desired in any particular installation, the lower working pressures of the present invention will allow for lower pressure pumps to be used, further increasing efficiency and reducing cost.

The direct thermal communication between the heat discharge member 26 and the evaporator 60 more efficiently cools servers 24 by more efficiently transferring heat from the heat discharge members 26 of the servers 24 to the evaporator 60. The improved heat transfer capabilities help provide for improved power efficiency of the data center 20. Power efficiency in the data center 20 may be measured with power unit effectiveness (“PUE”), which is a measurement of how efficiently a particular data center uses its power. PUE is calculated by dividing the total incoming power to the data center by the power delivered by the servers. The total incoming power to the data center includes the power delivered by the servers plus any electrical and mechanical support systems such as chillers and fans. Lower PUE values are better, as they indicate more incoming power is consumed by the servers instead of the support equipment. Traditional data centers typically have PUE values of about 2.5. In one exemplary embodiment, the cooling system 10 provides for a more power efficient data center 20 having a PUE value of about 1.03.

Still other variations and modifications are possible for the cooling system 10 according to the teachings of the present invention. For example, and with reference now primarily to FIG. 4, another embodiment 110 of the cooling system may comprise a plurality of evaporators 160 associated with corresponding server racks 122. In addition, the cooling system 110 may also be provided with a condenser 150 having a fan or blower system 155 operatively associated therewith. Fan or blower system 155 may be used to provide additional cooling air to condenser 150 to absorb additional heat 158 from the condensing working fluid 140 in the coils 156 of condenser 150.

The second embodiment 110 may also be provided with additional components and devices to help circulate working fluid 140 during certain operational modes and temperature regimes wherein the pressure differential resulting from the phase changes of the working fluid 40 will be insufficient to circulate the quantity of working fluid 140 required for the particular heat load. More specifically, the cooling system 110 illustrated in FIG. 4 may also comprise a compressor 182, a pump 180, and a liquid receiver 186. The compressor 182 may be provided in the return conduit 176, in parallel with a bypass valve 185. Pump 180 and liquid receiver 186 may be provided in the supply conduit 174, as depicted in FIG. 4. Compressor 182 may be operated in circumstances wherein the temperature of the cooling medium (e.g., air) provided at the condenser 150 is too high to fully condense the working fluid 140. When the compressor 182 is not needed, the working fluid 140 may bypass compressor 182 via bypass valve 185. Pump 180 may assist in returning the working fluid 140 in a liquid state 144 to the various evaporators 160, whereas liquid receiver 186 ensures that the working fluid 140 will be supplied to pump 180 in a liquid state 144.

As was the case for the first embodiment 10, the second embodiment 110 of the cooling system may be operated at pressures that are considerably lower than the operating pressures of conventional cooling systems. As a consequence, the compressor 182 and pump 180 need not be designed and configured to operate at high working pressures. In one embodiment, the compressor 182 may comprise an oil-less, magnetically-driven compressor having a variable frequency drive. Alternatively, other types of oil-less compressor systems that are now known in the art or that may be developed in the future could also be used. Pump 180 may similarly comprise a liquid pump suitable for low operating pressures. Ideally, compressor 182 and pump 180 should be of the type that will permit the working fluid 140 to remain substantially free of lubricant.

The system 110 may also comprise a liquid receiver 186 which, in one embodiment may be operatively associated with compressor 182. As mentioned above, the liquid receiver 186 separates the working fluid 140 in the liquid phase 144 from the working fluid 140 in the vapor phase 142. Maintaining a proper ratio of the working fluid 140 in the liquid phase 144 to the vapor phase 142 helps to maintain the circulation throughout the cooling system 110 and otherwise enhances the operation of the cooling system 110. That is, a sufficient amount of the working fluid 140 in the liquid phase 144 must be supplied to the evaporators 160 to insure adequate cooling of the servers 124. One factor that affects the amount of working fluid 140 in the liquid phase 144 is the ambient temperature of condenser 150 which affects the rate of condensation of the working fluid 140.

In one embodiment, the amount of working fluid 140 in the liquid phase 144 in the liquid receiver 186 is monitored by a sensor 187. If the working fluid 140 in the liquid phase 144 drops below a predetermined level, the sensor 187 signals the compressor 182 to assist in compressing the working fluid 140 to increase the amount of working fluid 140 that is condensed in condenser 150. If needed, additional amounts of working fluid 140 may also be drawn from a storage tank 188.

The system 110 may also be provided with a valve 184 provided at each inlet end of each evaporator 160. Valves 184 may be modulated to provide working fluid 140 in the liquid state 144 in an amount consistent with the heating load on the associated evaporator 160. For example, valve 184 can be opened to increase the amount of working fluid 140 delivered to the evaporator 160 for increased heat loads from the server 124. Conversely, lower heat loads on evaporator 160 will allow the valve 184 to be closed, thereby reducing the amount of working fluid 140 that is delivered to the evaporator 160.

Cooling system 110 may also be provided with additional components and devices, such as one or more temperature sensors 192 and pressure sensors 194. The temperature and pressure sensors 192 and 194 may be provided at various locations throughout the system 110 wherein it might be desired or required to monitor the temperature or pressure of the working fluid 140. A control system (not shown) may be operatively connected to the various components and devices of cooling system 110, such as, for example, the condenser fan system 155, pump 180, compressor 182, valves 184, check valve 185, and level sensor 187, to control the function and operation thereof. More specifically, the control system can then sense the various operational states, temperatures, and pressures of the system and operate the various components as required or desired to achieve a desired operational state or to accommodate certain changes or cooling demands on the system 110. For example, the control system may monitor and change the flow rate of the working fluid 140 throughout the cooling system 110 (e.g., via operation of the pump 180 and compressor 182). The control system may also operate the valves 184 to vary the flow rate of the working fluid 140 to the various evaporators 160, as needed.

Having herein set forth preferred embodiments of the present invention, it is anticipated that suitable modifications can be made thereto which will nonetheless remain within the scope of the invention. The invention shall therefore only be construed in accordance with the following claims: 

1. A cooling system for a server, comprising: an evaporator having an inlet end and an outlet end, said evaporator being in thermal communication with said server so that said evaporator absorbs heat generated by the server; a condenser having an inlet end and an outlet end, the inlet end of said condenser being fluidically connected to the outlet end of said evaporator, the outlet end of said condenser being fluidically connected to the inlet end of said evaporator; and a working fluid disposed within said evaporator and said condenser, said working fluid being substantially free of lubricant, said working fluid absorbing heat generated by the server in said evaporator, the absorbed heat changing said working fluid from a liquid phase to a vapor phase in the evaporator, said working fluid releasing heat in said condenser, the released heat changing said working fluid from the vapor phase to the liquid phase in said condenser, the phase changes of said working fluid in said evaporator and said condenser resulting in pressure changes sufficient to cause said working fluid to circulate between said evaporator and said condenser in a self-sustaining, phase-change thermodynamic cycle.
 2. The cooling system of claim 1, wherein said condenser is located at an elevation that is higher than an elevation of said evaporator.
 3. The cooling system of claim 1, wherein said working fluid changes from the liquid phase to the vapor phase at a temperature of about 18° C. at a pressure of about 1 bar.
 4. The cooling system of claim 3, wherein said working fluid comprises a halocarbon fluid.
 5. The cooling system of claim 4, wherein said halocarbon fluid comprises 1,1,1,3,3-pentafluoropropane.
 6. The cooling system of claim 1, wherein the server includes a heat discharge member and wherein said evaporator is in direct thermal communication with the heat discharge ember of the server.
 7. The cooling system of claim 1, wherein said evaporator and said condenser comprise fluid conduits therein, and wherein said fluid conduits comprise diameters sufficiently large to reduce a velocity of said working fluid flowing therein below a level at which a pressure head loss would prevent said working fluid from circulating through said cooling system in the self-sustaining, phase-change thermodynamic cycle.
 8. A method for cooling a server, comprising: evaporating a working fluid contained in an evaporator in thermal communication with the server, the working fluid being substantially free of lubricant, said evaporating producing a working fluid stream in a vapor state; supplying the working fluid stream in the vapor state to a condenser; condensing the working fluid in the vapor state to produce working fluid stream in a liquid state; and returning the working fluid stream in the liquid state to the evaporator, wherein phase changes of the working fluid in the evaporator and the condenser result in pressure changes sufficient to cause the working fluid to circulate between the evaporator and the condenser in a self-sustaining, phase-change thermodynamic cycle.
 9. The method of claim 8, wherein the working fluid stream in the vapor state and said the working fluid stream in the liquid state flow at velocities that are below a level at which a pressure head loss would prevent the working fluid from circulating through the evaporator and the condenser in the self-sustaining, phase-change thermodynamic cycle.
 10. The method of claim 8, wherein said evaporating is conducted at a pressure of about 1 bar.
 11. The method of claim 10, wherein said evaporating is conducted at a temperature of about 18° C.
 12. The method of claim 8, wherein said supplying comprises supplying the working fluid stream in the vapor state to the condenser that is located at an elevation that is higher than an elevation of the evaporator.
 13. A cooling system for a server having a heat discharge member, comprising: an evaporator having an inlet end and an outlet end, the evaporator being in direct thermal communication with the heat discharge member of the server; a condenser having an inlet end and an outlet end, the condenser being located at a higher elevation than the evaporator; a supply conduit connecting the outlet end of the condenser to the inlet end of the evaporator; a return conduit connecting the outlet end of the evaporator to the inlet end of the condenser; and a working fluid disposed within the evaporator, the condenser, the supply conduit, and the return conduit, wherein the working fluid enters the inlet end of the evaporator in a liquid phase and absorbs heat from the heat discharge member thereby transitioning the working fluid from the liquid phase to a vapor phase, the working fluid in the vapor phase exits the outlet end of the evaporator and travels though the return conduit before entering the inlet end of the condenser, wherein the working fluid in the vapor phase discharges heat to ambient thereby transitioning the working fluid in the vapor phase to the liquid phase before traveling through the supply conduit back to the evaporator.
 14. The cooling system of claim 13, wherein the working fluid has a boiling point of about 15° C. (60° F.) at about 1.01 bar (1 atm).
 15. The cooling system of claim 13 further comprising a pump connected to the supply conduit.
 16. The cooling system of claim 13, further comprising a controller operatively associated with the supply conduit, wherein the controller controls the supply of the working fluid to the evaporator.
 17. The cooling system of claim 13, further comprising a compressor connected to the return conduit.
 18. The cooling system of claim 17, wherein the compressor comprises a magnetically-driven compressor.
 19. The cooling system of claim 13, wherein the evaporator comprises a microchannel heat exchanger.
 20. The cooling system of claim 13, wherein the condenser comprises a microchannel heat exchanger.
 21. The cooling system of claim 13, wherein the condenser comprises a cooling tower.
 22. A method for cooling a server having a heat discharge member, comprising: supplying a working fluid in a liquid phase to an evaporator in direct thermal communication with the heat discharge member of the server; discharging heat from the heat discharge member of the server to the evaporator; transferring the discharged heat from the heat discharge member to the working fluid in the liquid phase, wherein the working fluid in the liquid phase absorbs the discharged heat; transitioning the working fluid into a vapor phase; supplying the working fluid in the vapor phase from the evaporator to a condenser located at a higher elevation than the evaporator, wherein the working fluid in the vapor phase is supplied from the evaporator to the condenser by a pressure differential caused by the condensation of the working fluid in the condenser; cooling the working fluid in the vapor phase with the condenser; transitioning the working fluid in the vapor phase to the liquid phase; and supplying the working fluid in the liquid phase back to the evaporator.
 23. The method for cooling a server of claim 22, wherein the working fluid has a boiling point of about 15° C. (60° F.) at about 1.01 bar (1 atm).
 24. The method for cooling a server of claim 22 further comprising: monitoring a temperature of the working fluid in the evaporator; and changing the supply of working fluid to the evaporator depending on the temperature of the working fluid.
 25. The method for cooling a server of claim 22, wherein supplying the working fluid in the liquid phase back to the evaporator comprises pumping the working fluid in the liquid phase from the condenser to the evaporator.
 26. The method for cooling a server of claim 22, wherein supplying the working fluid in the vapor phase from the evaporator to the condenser comprises pumping the working fluid in the vapor phase from the evaporator to the condenser.
 27. The method for cooling a server of claim 26 further comprising: monitoring a condensation temperature of the condenser and a temperature of the working fluid in the vapor phase; and bypassing the pumping of the working fluid in the vapor phase from the evaporator to the condenser when the condensation temperature is less than the temperature of the working fluid in the vapor phase.
 28. The method for cooling a server of claim 22 further comprising, storing the working fluid in a liquid receiver prior to supplying the working fluid to the evaporator. 